Stent retained on a balloon catheter

ABSTRACT

A balloon is inflated from a collapsed configuration, then deflated. A polymeric stem is then disposed over the deflated balloon and the stent crimped to the balloon.

The application is a divisional of U.S. application Ser. No. 13/252,965filed Oct. 4, 2011, which is a continuation of U.S. application Ser. No.11/864,834 filed Sep. 28, 2007, now U.S. Pat. No. 8,046,897, thecontents of which are hereby incorporated by reference in their entiretyfor all purposes.

TECHNICAL FIELD

This invention relates to methods of crimping a stent on a balloon, anda stent secured to a catheter assembly.

BACKGROUND

A stent, as illustrated in FIG. 1, is an intravascular prosthesis thatis delivered and implanted within a patient's vasculature or otherbodily cavities and lumens by a balloon catheter. The structure of astent is typically composed of scaffolding, substrate, or base materialthat includes a pattern or network of interconnecting structuralelements often referred to in the art as struts or bar arms. Referringto FIG. 1, an exemplary stent 14 is illustrated. Stent 14 can include aplurality of struts 16 connected by linking struts 11, with interstitialspaces 18 located in between the struts. The plurality of struts 16 canbe configured in an annular fashion in discrete “rows” such that theyform a series of “rings” throughout the body of stent 14. Stents can beused in percutaneous transluminal coronary angioplasty (PTCA) orpercutaneous transluminal angioplasty (PTA). Conventional stents andcatheters are disclosed by U.S. Pat. Nos. 4,733,665, 4,800,882,4,886,062, 5,514,154, 5,569,295, and 5,507,768. In advancing a stentthrough a body vessel to a deployment site, the stent must be able tosecurely maintain its axial as well as rotational position on thedelivery catheter without translocating proximally or distally, andespecially without becoming separated from the catheter. Stents that arenot properly secured or retained to the catheter may slip and either belost or be deployed in the wrong location. The stent must be “crimped”in such a way as to minimize or prevent distortion of the stent and tothereby prevent abrasion and/or reduce trauma to the vessel walls.

Generally, stent crimping is the act of affixing the stent to thedelivery catheter or delivery balloon so that it remains affixed to thecatheter or balloon until the physician desires to deliver the stent atthe treatment site. Current stent crimping technology is sophisticated.Examples of such technology which are known by one of ordinary skill inthe art include a roll crimper; a collet crimper; and an iris orsliding-wedge crimper. To use a roll crimper, first the stent is slidloosely onto the balloon portion of the catheter. This assembly isplaced between the plates of the roll crimper. With an automated rollcrimper, the plates come together and apply a specified amount of force.They then move back and forth a set distance in a direction that isperpendicular to the catheter. The catheter rolls back and forth underthis motion, and the diameter of the stent is reduced. The process canbe broken down into more than one step, each with its own level offorce, translational distance, and number of cycles. This processimparts a great deal of shear to the stent in a direction perpendicularto the catheter or catheter wall. Furthermore, as the stent is crimped,there is additional relative motion between the stent surface and thecrimping plates.

The collet crimper is equally conceptually simple. A standarddrill-chuck collet is equipped with several pie-piece-shaped jaws. Thesejaws move in a radial direction as an outer ring is turned. To use thiscrimper, a stent is loosely placed onto the balloon portion of acatheter and inserted in the center space between the jaws. Turning theouter ring causes the jaws to move inward. An issue with this device isdetermining or designing the crimping endpoint. One scheme is toengineer the jaws so that when they completely close, they touch and acenter hole of a known diameter remains. Using this approach, turningthe collet until the jaws abut each other crimps the stent to the knownouter diameter. While this seems ideal, it can lead to problems. Stentstruts have a tolerance on their thickness. Additionally, the process offolding non-compliant balloons is not exactly reproducible.Consequently, the collet crimper exerts a different amount of force oneach stent in order to achieve the same final dimension. Unless thisforce, and the final crimped diameter, is carefully chosen, thevariability of the stent and balloon dimensions can yield stent orballoon damage.

In the sliding wedge or iris crimper, adjacent pie-piece-shaped sectionsmove inward and twist, much like the leaves in a camera aperture. Thiscrimper can be engineered to have two different types of endpoints. Itcan stop at a final diameter, or it can apply a fixed force and allowthe final diameter to float. From the discussion on the collet crimper,there are advantages in applying a fixed level of force as variabilityin strut and balloon dimension will not change the crimping force. Thesliding wedges impart primarily normal forces. As the wedges slide overeach other, they impart some tangential force. Lastly, the sliding wedgecrimper presents a nearly cylindrical inner surface to the stent, evenas it crimps. This means the crimping loads are distributed over theentire outer surface of the stent.

All current stent crimping methods were developed for all-metal stents.Stent metals, such as stainless steel, are durable and can take abuse.When crimping is too severe, it usually damages the underlying balloon,not the metal stent. But polymeric stents present different challenges.A polymer stent requires relatively wider struts than metal stents so asto provide suitable mechanical properties, such as radial strength. Atthe crimping stage, less space is provided between the struts which canresult in worse stent retention than a metallic stent. Moreover, the useof high processing temperature during the crimping process to enhancestent retention may not be possible as a polymeric stent may have aglass transition temperature between 40-60 degrees Celsius. Higherprocessing temperatures may cause the stent to lose some of itspreferred mechanical properties.

Polymeric stents can also be more susceptible to crack propagationduring crimping or expansion by a balloon. When a polymeric stent isplaced on a balloon and a crimping pressure applied, the load on thestruts can vary significantly. A significant cause for this loadvariation is the balloon surface, which due to its variation in surfacegeometry and stiffness properties over the surface imposes non-uniformreaction forces to crimping against the stent luminal surface. As thestent is pressed into the balloon surface, different stent struts willexperience different loadings because the balloon does not everywherehave the same stiffness properties, i.e., some areas are less compliantthan others. Moreover, the balloon surface is generally not smooth. Assuch, bumps or mounds over the balloon surface, especially in areas ofrelatively high stiffness, can produce different loadings across thestent body. For example, when a stent is crimped on a non-compliantballoon having folded wings or pleats, there can be noticeable bumps ormounds in the areas where the material is folded. Moreover, thestiffness in these areas, i.e., where the balloon material is foldedover itself, can be noticeable higher than in areas distal from thefolds. Variations in the surface geometry of the balloon and/orstiffness about the circumference and/or length of the balloon can leadto high stress concentrations resulting in twisting, bending, warping ofindividual struts or segments of the stent. Excessive loading in areasnear corners, especially corners having surface imperfections createdduring the stent making process (typically a high stress area), canresult in propagation of micro cracks leading to a significant reductionin strength or outright failure of a strut when the stent is expanded toits full diameter by the balloon.

The present invention provides a novel method of crimping a stent, morespecifically a polymeric stent on an expandable member or a balloon, anda novel apparatus for delivery of a stent on a balloon catheter.

SUMMARY

In accordance with one embodiment, a method of crimping a stent on aballoon of a catheter assembly includes inflating a balloon, deflatingthe balloon, disposing the stent over the balloon after deflating theballoon, and then crimping the stent to the balloon. The balloon may beinflated to an intermediate pressure, maximum pressure, orhyper-inflated pressure for the balloon before being deflated. Variouscrimping procedures known in the art may be used. In some embodiments,the crimping procedure may include a step in which the balloon isre-inflated when the stent is disposed over the balloon. In someembodiments, the deflating step may deflate the balloon to aboutatmospheric pressure.

In some embodiments, the act of inflating the balloon inflates a balloonconfigured in a collapsed configuration, such that the balloon hasprearranged folds that are heat set. When the balloon is inflated, theheat set folds are removed, or substantially removed from the balloon.When the balloon is deflated, the balloon may partially reform thefolds.

In another embodiment, a method of securing a stent on a balloon of acatheter assembly, the balloon having pre-arranged folds when configuredin a collapsed configuration, includes the steps of inflating theballoon such that the folds are substantially or completely removed, andcrimping the stent to the balloon after the folds are substantially orcompletely removed. In some embodiments, the balloon is configured in acollapsed configuration, the folds are such as to form shape memory in amembrane of the balloon, and the crimping step includes crimping thestent to the balloon configured such that a portion of the shape memoryin the balloon is still present in the membrane. During the crimpingstep, the balloon may be compressed such that at least a portion of thefolds present in the collapsed configuration are replaced by irregularfolds.

According to another embodiment, a method of crimping a stent to aballoon includes the steps of inflating the balloon from an initialdiameter to a final diameter, deflating the balloon such that theballoon forms a relaxed state, and after deflating the balloon, crimpingthe stent to the balloon. In some embodiments, the balloon pressure isreduced to about atmospheric when the balloon is in the relaxed state.

According to another embodiment, a balloon catheter includes a balloonof the type having a membrane predisposed to form at least oneprearranged fold when in a collapsed configuration, the balloon beingconfigured in a compressed configuration such that the at least oneprearranged fold is replaced by at least one irregular fold in thecompressed configuration, and a stent secured to the balloon.

According to another embodiment, a balloon catheter includes a balloonof the type configured to have at least one prearranged fold when in acollapsed configuration, and a stent compressed onto the balloon suchthat the balloon is devoid of the at least one prearranged fold. In someembodiments, the balloon is a non-compliant balloon. In someembodiments, the balloon has a plurality of prearranged folds whenconfigured in a collapsed configuration, and when the stent iscompressed onto the balloon the plurality of folds are replaced by foldsresulting from the compression of the stent onto the balloon.

In one aspect, embodiments of a method and apparatus are disclosed thatcan reduce the instances of failed or bent stent struts during crimpingand/or subsequent expansion of the stent by a balloon catheter. Inanother aspect, the disclosure provides a method and apparatus toimprove stent retention and uniform expansion of the stent. In anotheraspect, the disclosure provides a method and apparatus for an improvedmanufacturing procedure, such as elimination of time consuming steps andimproved quality control when stents are secured to a balloon.

DESCRIPTION OF FIGURES

The figures have not been drawn to scale and portions thereof have beenunder or over emphasized for illustrative purposes.

FIG. 1 is a perspective view of a stent;

FIGS. 2A, 2B, 2C and 2D illustrate crimped stents that have strutsmisaligned by a crimping process.

FIGS. 3A and 3B show side and axial views, respectively, of a ballooncatheter with a balloon configured in a collapsed configuration;

FIGS. 4A and 4B show side and axial views, respectively, of the balloonwhen inflated.

FIG. 5 illustrates the balloon in FIG. 4A after having being deflated;

FIG. 6 illustrates a side view of the balloon of FIG. 5 with a stentdisposed over the balloon;

FIGS. 7 and 8 illustrate a crimping step for the stent to the balloonand the balloon and stent following crimping.

FIGS. 9A and 9B illustrate an alternative crimping procedure.

DESCRIPTION

Embodiments of the stent crimping methods of the invention disclosedherein are suitable to adequately and uniformly crimp a balloonexpandable stent onto a balloon or expandable member of a catheterassembly. The embodiments of the invention may also be applicable tostent-grafts. In one embodiment, the method of the present invention isparticularly directed to crimping of a biodegradable, polymeric stent ona balloon of a catheter assembly. A biodegradable polymer stent has manyadvantages over non-biodegradable metal stents, including the ability tobe disposed in the body only for the duration of time until the intendedfunction of the stent has been performed.

Crimping of polymer stents has proven more challenging than metalstents. A polymer, having generally less favorable stress/strainproperties than a metal, is more likely to become damaged duringcrimping as compared to a metal stent. High stresses are better handledby a metal stent due to its inherent material properties. Polymericstents, however, can fracture when faced with a similar crimp loading.As such, a polymer stent is generally less tolerant of localized highstresses resulting from non-uniform applied loads that can occur whenthe stent is pressed into a balloon.

FIGS. 2A, 2B, 2C and 2D illustrate four different polymeric stentscrimped to balloons. Ideally, all struts for this particular stentshould extend parallel to the longitudinal axis after crimping. However,in each of these figures one or more stent struts or rings has beentwisted, bent or misaligned in an desired way, i.e., deformed such thatstruts do not extend parallel to the longitudinal axis as intended. FIG.2A shows an over-compressed ring 19 a at the end of the stent in whichstruts abut each other and are twisted downward. The stent in FIG. 2Bshows a similar situation for the struts of ring 19 b, also located atthe end. Additionally, it can be seen in the stent of FIG. 2B that therings adjacent ring 19 b are twisted upward. In FIG. 4C it can be seenthat an entire row of struts 19 c are twisted upwards or downwards. Asmentioned above, it is preferred by design that all struts for thisstent extend parallel to the stent longitudinal axis when the stent iscrimped to the balloon. FIG. 2D shows a similar undesired deformity forring 19 d. Additionally, ring 19 e located closer to the center of thestent in FIG. 2D also has an desired twist in its struts. When thesestents are expanded by the balloon, some of the aforementionedmisalignment, twisting or bending may be undone. In other cases balloonexpansion may not straighten out these struts and/or rings. Worse yet,balloon expansion may cause one or more of the misaligned struts tofail. In these cases, it is believed that the above noted undesirabledeformations were due in large part to variations in surface contourand/or compliance characteristics of the balloon surface which receivedthe stent during crimping.

Retention of a polymer stent on a balloon of a delivery catheter whileit is passed through a body lumen has proven more challenging than thatof a metallic stent. Polymer stents can require wider struts than metalstents so as to provide suitable mechanical properties, such as radialstrength, for the stent. At the crimping stage, less space is providedbetween the struts which can result in worse stent retention than ametallic stent. Moreover, the use of high processing temperature duringthe crimping process to enhance stent retention may not be possible as apolymeric stent may have a glass transition temperature close to bodytemperature. Higher processing temperatures may cause the polymericstent to lose some of its preferred mechanical properties.

Additionally, stents can shift on a balloon during a crimping process.For example, if the balloon is inflated while the stent is disposed overthe balloon in a crimping machine, the stent longitudinal axis canbecome skewed from the balloon longitudinal axis, or the stent can shiftaxially. If this should occur, the stent and balloon will need to beremoved from the crimping machine, the stent re-centered and thecrimping protocol re-started. It is believed that this misalignment isusually due to non-uniform expansion characteristics of folded balloons.For example, some wings or pleats of a folded balloon may unfold fasterthan others. This can produce a net torque on the stent over its length,or unequal radial expansion when the balloon is viewed in a planeperpendicular to the longitudinal axis. Friction between layers orinconsistent pre-loading of folds when wings are formed are factorscontributing to uneven balloon expansion.

FIGS. 3A and 3B show side and cross-sectional views of an expandablemember, such as a balloon 10, integrated at a distal end of a catheterassembly 12. The catheter 12 includes a valve 24 to relieve balloonpressure and a pressure source (not shown). In some embodiments, theballoon 10 is intended to include any type of enclosed member such as anelastic type member that is selectively inflatable to dilate from acollapsed configuration to a desired and controlled expandedconfiguration. The balloon 10 should also be capable of being deflatedto a reduced profile or back to its original collapsed configuration.The balloon 10 can be made from any suitable type of material and can beof any thickness so long as the ability to crimp the stent onto theballoon and optimum performance capabilities of the balloon are notadversely compromised. Performance properties include, for example, highburst strength, good flexibility, high resistance to fatigue, an abilityto fold, and ability to cross and re-cross a desired region of treatmentor an occluded region in a bodily lumen, and a low susceptibility todefects caused by handling and crimping, among other possibilities.

The balloon is illustrated in FIG. 3A in a collapsed configuration. Thecollapsed configuration can be the configuration that is conventionallyused during the process of crimping of a stent on a balloon. In thecollapsed configuration, the balloon 10 includes no liquid or gas in theinternal chamber of the balloon 10. Such collapsed configuration can bethe configuration of introduction and navigation of the balloon 10 inthe vascular system of a patient and may be referred to as aconfiguration of the balloon in which the balloon has a minimum profile.The diameter 10 a of the balloon 10 is then the minimum diameter for theballoon. The balloon can be folded when in the collapsed configuration,which is the typical case for collapsed non-compliant balloons. Thefolds in a collapsed configuration are prearranged folds, meaning thatthe balloon material is folded according to a particular pattern ordesign intended to achieve an objective, e.g., a minimum profile. Thefolding is undertaken in an orderly manner either by hand or by amachine process. The folded parts of the balloons are often referred toas wings or pleats. The balloon is then typically heat set to hold thewings or pleats in place. For non-compliant balloons, which use materialthat is essentially non-elastic within the balloon operating ranges, theballoon inflates when wings or pleats have unfolded. As such,non-compliant balloons sometimes have several tightly wound layers ofprearranged folded balloon material when in the collapsed configurationin order to achieve a minimum profile or diameter for the balloon.

FIG. 3B depicts the balloon 10 folded in such a manner so as to createtwo opposing, prearranged wings or pleats 11, each having prearrangedkinks or folds 11 a, 11 b. In other embodiments, the catheter may have aballoon that has more than two wings. Balloons may be folded in a spiralor accordion like fashion, each approach to achieve a specificobjective, e.g., low profile, uniform expansion, reduced manufacturingcomplexity or quality control. Once folded, the balloon 10 is heat setso that folds 11 are maintained. The heat set can be such that if theballoon pressure is increased enough to unfold the wings, and then theballoon pressure is reduced back to below atmospheric, the balloon willtake the same shape as it had prior to inflation.

Embodiments of the invention include the known types of collapsedballoon configurations, provided they are capable of being crimped to astent and then delivered to a site within the body according to thedisclosure. Stents crimped to compliant and semi-compliant balloons mayalso benefit from the teachings of the invention. As such, stentdelivery systems that use compliant and semi-compliant balloons are alsoconsidered within the scope of the invention. Balloon folding techniquesand balloon types are discussed in greater detail in U.S. Pat. Nos.5,556,383, 6,488,688 and U.S. Pub. No. 2005/0244533, the entire contentsof which are incorporated by reference as examples of the state of theart in this area.

Methods according to some embodiments of the invention can include aninflation of the balloon from a collapsed configuration to an inflatedstate, a deflation of the balloon from the inflated state, and thencrimping the stent to the balloon using any known crimping procedure,provided those procedures are not inconsistent with the disclosure. Thesuitable choices for the crimping device and/or procedure that may beused in connection with embodiments of the invention will be understoodfrom the disclosure. The crimping, inflation and deflation aspects ofthe disclosure may each include additional steps or processes, some ofwhich are disclosed herein while others would be readily apparent basedon the disclosure.

According to a disclosed embodiment, a method for securing a stent to aballoon begins with inflation of the balloon 10 to an inflated state, asshown in FIG. 4A. In this state, the balloon 10 has an inflated diameter10 b. In some embodiments, all balloon folds are removed, substantiallyall balloon folds are removed, the balloon is fully unfolded or theballoon is substantially fully unfolded in the inflated state. In someembodiments, the balloon takes its final expanded shape, e.g.,cylindrical, or substantially its final expanded shape, when the balloonis in the inflated state.

The inflated shape may also be understood in terms of the internalballoon pressure when the balloon is in the inflated state, instead ofthe balloon shape. The balloon 10 is inflated by a pressure source (notshown) connected to the proximal end of the catheter 12 in FIG. 4A. Theinflated state may be achieved after the balloon is inflated to amaximum intended pressure or design pressure, an intermediate pressure,a hyper-inflated pressure. In some embodiments, a balloon inflated toany pressure that causes the balloon to at least partially expand placesthe balloon in an inflated state. Such embodiments include those inwhich the pressure is sufficient to break any friction between foldedwings in the balloon material. A maximum intended pressure refers to aballoon pressure considered the highest from a standpoint of acceptablerisk of damage to the balloon as prescribed under a standard medicalprotocol. A balloon manufacturer can supply this information or it canbe determined using known methods. The maximum intended pressure mayalso refer to the diameter of the balloon when fully expanded, e.g.,diameter 10 b in FIG. 4A. In the case of non-compliant balloon thiswould correspond to the outer balloon diameter after the wings have beenfully unfolded. The maximum intended pressure may also refer to theballoon pressure that produces approximately the largest diameterdesignated for clinical use, in the case of compliant or semi-compliantballoons.

In some embodiments, the inflated state may correspond to ahyper-inflated balloon. Hyper-inflation is defined as any pressure orsize above the intended expanded configuration but less than a pressureor size that creates an unacceptable risk that the balloon will bedamaged. For example, a hyper-inflated pressure for a 3.0 mm balloonwould be an applied balloon pressure that causes the balloon to expandto 3.5 mm or 4.0 mm, which diameters should usually not cause balloondamage. Balloon diameter tolerances depend on the type of balloon andthe material from which the balloon is made, among other factors. Themanufacturer of the balloon can provide such information to a user, forexample.

An intermediate pressure refers to a pressure that, while not reachingthe maximum intended pressure, is sufficient to substantially remove thefolds or kinks in the balloon material when the balloon was in thecollapsed configuration, or places the balloon in a substantiallyunfolded configuration. For example, an intermediate pressure for theballoon 10 would be the pressure sufficient to substantially undo thekinks 11 a and 11 b caused by the folded, collapsed configuration inFIG. 3B, but less than the maximum intended pressure for balloon 10.FIG. 4B depicts the balloon 10 when inflated to an intermediatepressure. Thus, FIG. 4B is one example of a balloon were the folds aresubstantially removed. The places on the balloon material where theballoon material was folded over itself, i.e., kinks or folds 11 a, 11b, cannot be identified. Also, the balloon has taken an essentiallycircular shape.

A softening agent can be applied to the balloon surface before, duringor after the inflated state has been reached. This can make the balloonsurface softer or more pliable and can also help to work-out some of theshape memory properties of the balloon material remaining from the heatset. The term “work-out” is intended to mean the application of aloading, e.g., inflating, coating, and/or heating of the balloon toreduce or undo the shape memory in the material that resulted from theballoon folds in the collapsed configuration, such as folds that wereheat set in place. The balloon may be inflated and deflated severaltimes as this may also help to work-out the shape memory properties. Thedeflated pressure for these embodiments may be near the collapsedconfiguration or relatively close to the less-than the intended maximumpressure, the maximum pressure or the hyper-inflated pressure level. Forexample, the inflated pressure of the balloon may be cycled betweenambient pressure and the intended maximum pressure, the intermediatepressure and the maximum pressure, etc.

The temperature of the liquid or gas used to expand the balloon to theinflated state may be adjusted to other than ambient or roomtemperature, and the inflated state may be maintained for a period oftime. For example, the balloon may be expanded or contracted by aheated, chilled or cold fluid, respectively. In some embodiments, aheated fluid can be defined as above 25 deg. C. In some embodiments, thetemperature can be below 200 deg. C., or alternatively below 150 deg.C., or alternatively below 100 deg. C., or alternatively below 75 deg.C. In some embodiments, the temperature can be between 25 deg. C. and100 deg. C. A cooled fluid can mean below 25 deg. C. A chilled fluid canmean below 0 deg. C.

Subsequent to configuring the balloon 10 in an inflated state, theballoon is placed in a relaxed state, as depicted in FIG. 5. For thisembodiment, the balloon 10 has a maximum radial extent 10 d, a surfacehaving random depressions 10 e and irregular or random folds 10 c. Arelaxed state is intended to mean the shape that the balloon takes afterthe pressure in the balloon is reduced from the inflated state. In someembodiments the relaxed state is such that when the stent is crimped tothe balloon, new folds or creases replace prearranged folds from thecollapsed configuration. In some embodiments, the balloon is maintainedat a pressure, such as greater than about one atmosphere for a relaxedstate. In this case, the stent may be crimped onto the balloon while theinternal pressure is allowed to release.

In some embodiments, the relaxed state corresponds to the balloonsurface showing generally the same wings that it had in the collapsedconfiguration as a result of shape memory in the membrane, but the wingsare partially formed. When the stent is subsequently crimped to theballoon, irregular or random folds form as a result of thecircumferential forces during crimping, in spite of the presence of thepartially reformed wings. In some embodiments, some of the prearrangedfolds can be found on the balloon after crimping. In some embodiments,the prearranged folds are no longer present after crimping.

For example, the relaxed state for balloon 10 can correspond to a shapethat shows a partial formation of the two spiral-wound wings 11 in FIG.3B, but when the stent is crimped to the balloon 10 those wings 11 areno longer present. Instead, the prearranged folds are replaced byirregular or random folds 22 a that form as the stent 14 bears down onthe balloon during crimping, as depicted in FIG. 8. Irregular or randomfolds or wrinkles is intended to mean folds or wrinkles that are notformed in a prearranged manner. In some embodiments, irregular or randomfolds or wrinkles, e.g., folds 22 a (FIG. 8) or 10 e (FIG. 5) describethe appearance of the balloon when left to form folds or wrinkles,either as the balloon membrane collapses under its own weight, e.g.,folds 10 e in FIG. 5, or when prearranged folds from the collapsedconfiguration are replaced by new folds or wrinkles during crimping,e.g., folds 22 e in FIG. 8. In some embodiments the surface of theballoon may have substantially all, or only a portion of the folds fromthe collapsed configuration present in the relaxed state. In someembodiments, the relaxed state is such that when the stent is crimped tothe balloon, heat set folds are not reformed when the stent is crimpedto the balloon. Rather, new creases or folds replace the heat set folds.

A balloon pressure for the relaxed state may be arrived at by simplyopening the valve 24 to allow fluid to exit from the balloon, or by acontrolled release of the fluid. The fluid may be released continuouslyor in stages, e.g., 1 atm per minute, etc. In some embodiments, arelaxed state corresponds to the shape of the balloon when the pressureinside is at about atmospheric pressure. In other embodiments therelaxed state is achieved when the balloon pressure is decreased to apressure less than the pressure corresponding to the inflated state,e.g., 10-30%, less than 50%, 50-60%, 60-80%, 80-90%. In someembodiments, the balloon pressure may be less than the maximum intendedpressure but higher than ambient pressure. In some embodiments, theballoon is allowed to deflate only by an amount sufficient to enable astent to be easily placed over the balloon, or so that a stent may besnugly fit over the balloon.

In some embodiments, the aforementioned inflating and deflating stepsmay be repeated one or more times in order to arrive at a desiredrelaxed state. These additional steps, in addition to other loadings ofthe balloon intended to work-out shape memory in the material, asdiscussed earlier, may be advantageous to further promote a more uniformexpansion of the balloon. A more uniform expansion of the balloon isdesirable because the balloon should be less likely to expand the stentin a irregular manner. Further, a more uniform expansion should equateto more uniform stiffness properties of the balloon over its surfaceand, in particular, in those areas proximal to bumps or mounds ascompared to areas distal from bumps or mounds. As such, when the stentis pressed into the balloon during crimping the localized stressconcentrations should not be as severe. This should reduce instances ofcrack propagation in a stent strut and/or undesired plastic deformationsof the crimped stent.

After the deflating step, the stent is placed over the balloon, which isin a relaxed state. This stage of the process is depicted in FIG. 6. Thestent has struts 16 separated by gaps 18. As indicated above, in someembodiments the balloon is inflated to atmospheric pressure or aboveatmospheric when in a relaxed state. The balloon may also be filled witha liquid when in the relaxed state. The balloon pressure may be up to anamount that allows the stent to be loosely placed on the balloon orsnugly fit on the balloon. FIG. 7 depicts the stent 14 on the balloon10, and the balloon 10 in a relaxed state when in a crimping device 20.The crimping device is designed to apply uniform radial compressiveforces 21 to the outer stent surface sufficient to plastically deformthe stent. The effect of the applied force 21 is shown in FIG. 8. Asdepicted in FIG. 9, the balloon 10 has irregular or random folds 22 abut not the prearranged folds 11 present in the balloon's collapsedconfiguration (FIGS. 3A and 3B).

The stent 14 may be fully crimped onto the balloon 10 in one step, orthe stent 10 may be crimped on the balloon using any multi-step protocolknown in the art provided it can be used or adapted for use consistentwith the teachings of the invention. Crimping can be defined as theprocess of mounting, fastening or securing a stent on a balloon. Thestent can then be fixedly carried by the balloon and deployed byinflation and subsequent withdrawal of the balloon in order to implantthe stent at a target site, such as a region of stenosis. The crimpprocess can include selectively, radially compressing or applyingpressure for positioning a stent on a balloon of a catheter assembly oran expandable delivery member of a catheter assembly. The compression orradial pressure during crimping can be segmented or uniform across thelength and/or circumference of the stent. The application of pressure bythe crimping device 20 can be continuous or applied in an intermittentor step-wise fashion. In these embodiments, the balloon can be deflatedand re-inflated until a final crimp configuration has been achieved.

The stent may be partially crimped on the balloon, followed by theballoon pressure being increased. The balloon may also be heated prioror during crimping. The final configuration of the stent 14 as fullycrimped is illustrated in FIG. 8. Protrusions 22 of the balloon aredisposed in the gaps 18 of the stent struts 16. It has been found thatin some embodiments, as a result of the aforementioned effects onballoon stiffness resulting from the disclosed inflation and deflationsteps, i.e., more uniform stiffness properties, a stent's slipresistance from the balloon following crimping can be improved. In theseembodiments, the balloon material is more capable of protruding into thegaps 18 before or during crimping. And when disposed in the gaps 18, theprotrusions 22 may be used to increase the slip resistance of the stent.In some embodiments, balloon material may more readily slip into gaps 18by maintaining the balloon 10 in an inflated state while the stent iscrimped. The valve 24 is regulated during crimping in order to relievethe balloon 10 pressure at a suitable rate as the stent continues tobear down on the balloon. For example, the pressure may be relieved atsuch a rate as to allow the balloon material sufficient time to fill orslip into the gaps but without increasing the balloon pressure beyond asafe limit. In one embodiment, the protrusions 22 preferably do notextend beyond the outer surface of the struts 16, while in otherembodiments the protrusions 22 may extend beyond the outer surface ofthe struts 16. In some embodiments, the balloon may be heated to promoteexpansion of the balloon material into gaps 18 before or duringcrimping. When the balloon material is disposed within the gaps 18, theballoon wall or membrane may become wedged, lodged, squeezed, or pinchedbetween the struts 16 when the crimping process is completed.

In some embodiments, the crimping device can hold the balloon pressureat a desired pressure or temperature for a period of time prior torelief of pressure. The process of crimping can also include, unlessotherwise specifically indicated, modifications made to the stent and/orballoon prior, during or subsequent to the application of crimpingpressure that is directed to retention of the stent on the balloon. Forexample, the balloon can have an adhesive coating to improve theretention of the stent on the balloon. In some embodiments, the ballooncan be dipped into a fluid or solvent such as acetone before sliding thestent on the balloon in order to soften the balloon material. This makesit easier for the balloon material to squeeze into the space between thestent struts. The solvents, such as acetone, may also partially dissolvethe surface of the stent or coating on the stent allowing for betteradhesion between the stent and the balloon. In some embodiments, asoftening fluid can be used that is a non-solvent for the stent or thecoating on the stent.

During the crimping stage, as with the inflation stage, the balloon 10can be inflated by application of a fluid or a gas at a temperatureother than ambient. In one embodiment, a heated fluid or gas is usedwhen the stent is crimped onto a deflating balloon. In some embodiments,heated can be defined as above 25 deg. C. In some embodiments, thetemperature can be below 200 deg. C., or alternatively below 150 deg.C., or alternatively below 100 deg. C., or alternatively below 75 deg.C. In some embodiments, the temperature can be between 25 deg. C. and100 deg. C. In some embodiments, the temperature is equal to or abovethe glass transition temperature (Tg) of a polymer of the stent body ora polymer of the stent coating (if applicable). In some embodiments, thetemperature is equal to or above Tg but less than a melting temperatureof a polymer of the stent body or a polymer of coating over the stentbody. In some embodiments, a cooled or chilled fluid or gas can be usedto inflate the balloon. Cooled can mean below 25 deg. C. Chilled canmean below 0 deg. C.

In some embodiments, the outer surface of the balloon or the innersurface of a stent can include a coating, such as an adhesive coating, adrug delivery coating, a protective coating, a polymeric coating. If itis desirable instead to increase, not decrease friction, a heatactivated adhesive coating may be used. For example, a polymer having amelting or glass transition temperature below that of a stent andballoon may be applied to the stent or balloon surfaces, and the stentand balloon heated to activate the coating so that the friction betweenstent and balloon is increased.

In another embodiment, the stent may be partially crimped and theballoon expanded by application of heat or inflated into the stent sothat the balloon material begins to protrude into the gaps 18 prior to afinal crimping step. FIG. 9A illustrates the balloon 10 and the stent 14in the partially crimped configuration. FIG. 9B illustrates the finalcrimped configuration for this embodiment. It has been found that whenthe aforementioned inflating and deflating steps are carried out priorto this crimping protocol, the stent does not become skewed relative tothe catheter or shifted axially along the balloon. The explanation forthis behavior is understood in light of the previous discussion.

The stent body itself is preferably made from a polymeric material suchas one or a combination of polymers. In some embodiments, such body canbe made from a combination of polymeric and metallic material(s). Insome embodiments, the stent is biodegradable. Both polymers and metallicmaterials can be biodegradable. In one preferred embodiment, the stentis completely or exclusively made from a polymeric material orcombination of polymeric materials, more specifically biodegradablepolymer(s). A polymeric stent can include some metallic components forallowing the stent to be viewed during the procedure; however, theamount of material is insignificant, does not impart any structuralfunction to the stent, or for viewing means only such that the stent isin essence made from a polymeric material or combination of polymers asis understood by one having ordinary skill in the art. In someembodiments, metallic stents are completely excluded from any of theembodiments of this invention. Metallic stents have a stent body (i.e.,struts or structural elements) made mostly or completely from a metallicmaterial such as a pure metal or an alloy. It should be noted thatbiodegradable is intended to include bioabsorbable, bioerodable, etc.unless otherwise specifically indicated.

In some embodiments, the stent can include a drug coating. The coatingcan be a pure drug or combination of drugs. The coating can include apolymeric carrier for the drug, either a single polymer or multiplepolymers. The coating can be layered as is understood by one of ordinaryskilled in the art.

The stent or the coating can be made from a material including, but arenot limited to, poly(N-acetylglucosamine) (Chitin), Chitosan,poly(hydroxyvalerate), poly(lactide-co-glycolide),poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate),polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide),poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid),poly(D,L-lactide), poly(caprolactone), poly(trimethylene carbonate),polyester amide, poly(glycolic acid-co-trimethylene carbonate),co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules(such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronicacid), polyurethanes, silicones, polyesters, polyolefins,polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymersand copolymers other than polyacrylates, vinyl halide polymers andcopolymers (such as polyvinyl chloride), polyvinyl ethers (such aspolyvinyl methyl ether), polyvinylidene halides (such as polyvinylidenechloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics(such as polystyrene), polyvinyl esters (such as polyvinyl acetate),acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides,polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, celluloseacetate, cellulose butyrate, cellulose acetate butyrate, cellophane,cellulose nitrate, cellulose propionate, cellulose ethers, andcarboxymethyl cellulose. Another type of polymer based on poly(lacticacid) that can be used includes graft copolymers, and block copolymers,such as AB block-copolymers (“diblock-copolymers”) or ABAblock-copolymers (“triblock-copolymers”), or mixtures thereof.

Additional representative examples of polymers that may be especiallywell suited for use in fabricating or coating the stent include ethylenevinyl alcohol copolymer (commonly known by the generic name EVOH or bythe trade name EVAL), poly(butyl methacrylate), poly(vinylidenefluoride-co-hexafluororpropene) (e.g., SOLEF 21508, available fromSolvay Solexis PVDF, Thorofare, N.J.), polyvinylidene fluoride(otherwise known as KYNAR, available from ATOFINA Chemicals,Philadelphia, Pa.), ethylene-vinyl acetate copolymers, and polyethyleneglycol.

A metal stent may also be used in conjunction with the above-describedembodiments. The stent can be made of a pure metal or a metal alloy suchas, but not limited to, stainless steel (316L), “MP35N,” “MP20N,”tantalum, nickel-titanium alloy, platinum-iridium alloy, gold,magnesium, or combinations thereof. “MP35N” and “MP20N” are trade namesfor alloys of cobalt, nickel, chromium and molybdenum available fromstandard Press Steel Co., Jenkintown, Pa. “MP35N” consists of 35%cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consistsof 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum. Stents madefrom bioabsorbable or biostable polymers could also be used with theembodiments of the present invention.

EXAMPLE

Polymeric stents were crimped onto balloon catheters using a procedureconsistent with the disclosure (hereinafter the “new crimping method”).For this test, a 3.0 mm non-compliant balloon was dilated from acollapsed configuration to an inflated state of between 12-20 psi, thendeflated so that the stent internal pressure was about equal toatmospheric pressure. In this relaxed state, a laser cut polymeric stent(OD=0.136 in) was placed on the balloon and aligned between two markerbands. The stent and balloon were then placed into a crimping machine.The stent was then crimped to a designated diameter (0.053 in) in onestep (from 0.136 in to 0.053 in) or several steps (from 0.136 in to 0.08in, then to 0.053 in) in the range of 30 to 50 degrees Celsius. Thecrimping procedure included the steps of inflating the balloon beforecrimping, and after the stent was partially crimped to 0.08 in. Thecrimped stent was then sterilized by electronic beam radiation with adose of about 25 kGray. The above procedure was performed for 30 stents.Only one of the crimped stents were observed to have very slight bentstruts at one end after being crimped using the new crimping method, asobserved under an optical microscope with 4× magnification. As a controlgroup, a different lot of the 30 laser cut stents were crimped onto 3.0mm non-compliant balloons using the same crimping procedure as above,but without inflating and deflating the balloon before crimping. In thisgroup, seven stents with bent struts or uneven alignment were observedunder the optical microscope at 4× magnification.

Ten stents crimped using the new crimping method, and ten stents fromthe control group were expanded by the balloon and subjected toaccelerated aging (40 degrees Celsius) at 24 hour and 65 hour timepoints. The expanded stents from both groups were then inspected forcracks or fractures after being expanded by the balloon. For purposes ofthis test, the stents with designed expanded diameters of 3.0 mm wereover-expanded to 3.5 mm, and then 4.0 mm diameters. TABLES 1 and 2 showthe number of cracked and broken stents found in the stents crimpedaccording to the new crimping method (“Lot B”) versus the stents crimpedfor the control group (“Lot A”).

TABLE 1 Expansion Results (24 hour accelerated aging time point)Expansion Deployed to 3.5 mm Deployed to 4.0 mm Sample # 25% 50% Broken25% 50% Broken Lot A 6 2 0 0 4 2 1 28 2 1 0 2 1 3 29 3 0 0 4 1 0 34 13 70 16 7 2 36 4 0 0 6 1 0 Lot B 26 2 0 0 5 1 0 45 5 0 0 7 0 0 46 2 0 0 4 10 47 0 0 0 2 0 0 48 0 0 0 0 0 0

TABLE 2 Expansion Results (65 hour accelerated aging time point)Expansion Deployed to 3.5 mm Deployed to 4.0 mm Sample # 25% 50% Broken25% 50% Broken Lot A 5 5 0 1 7 2 3 20 5 3 0 9 2 7 24 7 4 0 4 1 9 32 8 40 10 1 7 33 3 1 0 5 2 2 Lot B 27 2 0 0 5 0 0 34 8 0 0 9 0 0 38 4 0 0 4 11 40 8 3 0 5 3 5 51 5 0 0 7 0 0

“Sample #” indicates the particular sample from the respective lot.“25%” and “50%” refers to the length of a crack observed in a strut fora sample (as a percentage of the strut width). “Broken” indicates thenumber of broken struts following expansion for a sample. As evidentfrom the data shown in TABLES 1 and 2, the new crimping methodsignificantly reduces instances of cracked or broken struts after thecrimped stent was expanded by the balloon, as compared to the controlgroup.

TABLE 3 shows results of performance tests for the stents crimped usingthe new crimping method (Lot B) compared to the control group (Lot A).The data presented in this table show that the new crimping method doesnot produce any adverse effects on retention or slip resistance, recoil,radial strength and modulus of the stents as compared to the controlgroup. The stent retention test was evaluated by a dislodgement test.Two out of the five of stents crimped using the new crimping method werefound to have a much higher holding force.

TABLE 3 Recoil, retention, radial strength and modulus of stents LOT ALOT B # of # of Test samples Average STD Dev Test samples Average STDDev Recoil 5 8.6% 2.1% Recoil 5 7.9% 1.5% % Length 3.9% 2.7% % Length2.6% 0.8% Change Change Uniformity 0.04 0.03 Uniformity 0.06 0.01 ofExpansion of Expansion Radial 5 7.978 0.290 Radial 5 7.726 0.315Strength Strength Modulus 657.7 105.246 Modulus 677.9 85.334Dislodgement 5 1.2566 0.192361119 Dislodgement 5 1.2834 0.085909 testtest

What is claimed is:
 1. A method for making a medical device, comprising:providing a balloon having a shape memory forming prearranged pleats orwings, wherein the prearranged pleats or wings have kinks; working-outthe shape memory including removing the kink of at least one of theprearranged pleats or wings, thereby replacing the at least one of theprearranged pleats or wings with random folds; and crimping a stentincluding plastically deforming the stent onto the balloon while theballoon has the random folds.
 2. The method of claim 1, wherein thestent comprises a polymer having a glass transition temperature ofbetween 40 and 60 degrees Celsius.
 3. The method of claim 1, wherein theworking-out the shape memory includes removing at least one heat setpleat or wing that is folded in a spiral or accordion-like fashion. 4.The method of claim 3, wherein the balloon shape memory is worked-out byinflating and deflating the balloon or applying a fluid to a surface ofthe balloon.
 5. The method of claim 1, wherein when the stent is crimpedto the balloon all of the prearranged pleats or wings are replaced byrandom folds.
 6. The method of claim 1, wherein the balloon is a 3.0 mmballoon; and wherein the crimped stent has an outer diameter of 0.053inches.
 7. The method of claim 1, wherein the balloon is a non-compliantor semi-compliant balloon.
 8. The method of claim 1, wherein a membraneof the worked-out balloon is folded over itself in an irregular mannerwhen the stent has a final crimping diameter.
 9. The method of claim 1,wherein the stent is crimped to the balloon using an iris-type crimpingmechanism.
 10. (A method for making a medical device, comprising:providing a balloon having a shape memory forming prearranged folds;working-out the shape memory, whereby at least one of the prearrangedfolds is replaced by random folds; placing a stent over the balloonafter working out the shape memory, wherein the balloon is at leastpartially inflated when the stent is placed over the balloon; andcrimping the stent including plastically deforming the stent onto theballoon while the balloon has the random folds.
 11. The method of claim10, wherein the balloon is inflated such that the stent is snugly fitover the inflated balloon when the stent is placed over the inflatedballoon.
 12. The method of claim 10, wherein the balloon is inflatedsuch that the stent is loosely fit over the inflated balloon when thestent is placed over the inflated balloon.
 13. The method of claim 1,wherein the working out the shape memory includes inflating anddeflating the balloon several times.
 14. The method of claim 1, whereinthe crimping step includes using an iris crimper to plastically deformthe stent onto the balloon, wherein the iris crimper applies acontinuous crimping force until the stent reaches a final crimpdiameter.
 15. The method of claim 1, wherein the crimping step includesusing an iris crimper to plastically deform the stent onto the balloon,wherein the iris crimper applies a crimping force in an intermittentstep-wise fashion until the stent reaches a final crimp diameter. 16.The method of claim 1, wherein the stent is a laser cut polymeric stentmade from a material including poly (L-lactide).
 17. A method for makinga medical device, comprising: providing a balloon having a shape memoryforming prearranged pleats or wings, wherein the prearranged pleats orwings have kinks; working-out the shape memory by increasing a pressureuntil the balloon is inflated, wherein when the balloon is inflated allof the kinks are substantially removed, followed by placing the balloonin a relaxed state wherein at least one of the prearranged pleats orwings is replaced by random folds; and crimping the stent includingplastically deforming the stent onto the balloon while the balloon hasthe relaxed state.